Th@C₂(14)-C₈₆ and Th@C₃(18)-C₈₆: two missing C₈₆ isomers stabilized by the encapsulation of thorium

Two missing cage isomers of C-86 have been successfully stabilized by the encapsulation of the actinide ion Th. The two novel isomers of Th@C-86 were synthesized using the arc-discharge method and characterized using mass spectrometry, single-crystal X-ray crystallography, UV-vis-NIR spectroscopy and DFT calculations. The molecular structures of the two isomers were unambiguously assigned to Th@C-2(14)-C-86 and Th@C-3(18)-C-86 by crystallographic analysis. Both of these cage isomers of C-86 have been predicted by computational studies but have never been experimentally confirmed until now. DFT computation reveals that both Th@C-2(14)-C-86 and Th@C-3(18)-C-86 present closed-shell electronic configurations with formally Th(iv), consistent with other previously reported Th based mono-metallofullerenes. Th@C-2(14)-C-86 has emerged as the third lowest-energy isomer with significant predicted abundance at temperatures of fullerene formation. Th@C-3(18)-C-86, on the other hand, represents an unexpected isomer with low thermodynamic stability, but significant abundance. Its formation was further rationalized as a kinetically trapped intermediate obtained from the lowest-energy and abundant Th@C-2(8)-C-84, by a C-2 insertion and a single Stone-Wales transformation. This study highlights the unique impact of actinides on the stabilization of fullerene cage isomers and also provides a more in-depth understanding into the stabilization mechanism of fullerene cage isomers.